1. Field of the Invention
The present invention generally relates to gas generators employed in small rocket engines and, more particularly, is concerned with a monolithic high activity catalyst bed for use in a catalytic gas generator of a rocket engine.
2. Description of the Prior Art
Small rocket engines of the type utilized in space satellites are generally of monopropellant design employing a catalytic gas generator within a thrust chamber to convert a liquid propellant, such as hydrazine, into a high-temperature, propulsive power-producing gas. The gas generator has a catalyst bed in communication with the thrust chamber of the engine. The liquid propellant is injected into the catalyst bed where it reacts to produce the high-temperature gas. The gas exits from the catalyst bed and is expanded through a nozzle on the aft end of the chamber to produce thrust. U.S. Patents to Eggers et al (3,695,041) and Daly (4,352,782), assigned to the same assignee the present invention, disclose representative examples of this type of small rocket engine.
As recognized in the above-cited Daly patent, the primary life-limiting component of such monopropellant rocket engines is the catalyst used for decomposing the propellant into a high-temperature gas. A catalytic gas generator containing a bed of Shell 405 catalyst for reacting hydrazine propellant is limited to applications where the acceleration and shock environments are low. When stresses exceed approximately 250 psi, the Shell 405 catalyst begins to fail in compression and shatters into very small particles. After the catalyst has shattered, the gas generator may catastrophically fail or operate in a degraded manner.
Catalyst made with alternate Al.sub.2 O.sub.3 carriers have been tested and found to be two to three times stronger in compression than Shell 405 while possessing equivalent catalytic activity. The increased strength improves the resistance to environmental stress but the catalyst will not survive extreme environments. A significant effort has been made to utilize metal substrates that are coated with irridium. All efforts to date have been unsuccessful. The activity of the coated metals is not sufficient to decompose hydrazine. A partially successful technique consisted of coating a metal foam with Al.sub.2 O.sub.3 carrier, and then impregnating the Al.sub.2 O.sub.3 with irridium. A catalyst bed composed of this material was sufficiently active that hydrazine decomposed at low flow rates. However, the bed failed when the flow was increased to a practical level.
Consequently, a need still exists for a catalyst bed construction capable of withstanding and surviving severe acceleration and shock environments.